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April 3, 2010

Amidst all the scientific studies, academic research and political rhetoric, the debate continues on how much effect converting to electric cars – powered from our existing electrical grid – will have on the emissions of greenhouse gasses – CO2.

With the proliferation of these studies, some tracing CO2 contributions all the way back to the first exhalation of breath from the dinosaurs, it can be hard for the average rocket scientist, let alone mere mortal, to come to a conclusion.

Therefore, I have decided to shun the scientific calculator, eliminate the variables, and do a simple, empirical analysis of their direct impact.

First, the Coal burning Power Plants.

We are fortunate here in that someone, most likely math challenged, actually had the good sense to measure the CO2 output of power plants. It has been measured and confirmed many times since. Ends up the average Coal burning power plant emits 2.17 pounds of CO2 for every kilowatt-hour it produces. (EIA) Now, coal makes up only 48% of our electrical generation. Natural Gas produces 20% and it also emits CO2 (1.4 pounds per kWh). But, 30% comes from nuclear and renewable sources. So, taken together, our national electrical supply generates, on the high side, 1.51 pounds of CO2 per kWh.

Now for the Cars.

Basic Chemistry tells us that burning 1 gallon of gasoline emits 19.4 pounds of CO2. It also tells us, based on BTU content, there are 36 kWh of energy in that gallon of gas.

One place where all the research gets bogged down is trying to account for the varying efficiencies of our automotive contraptions. Automotive IC (Internal Combustion) engines are anywhere from 15 to 28% efficient in average driving. Well, for our worst case, empirical study, I am going to assume the most efficient – 28%. The most efficient means it is the most work we get for every globule of CO2 emitted. Indeed, the meaning is that 28% of the BTUs in that gallon of gas will get turned into useful, mechanical work. All the rest are wasted as heat. There are 124,000 BTU’s, give or take, in a gallon of gas, so at 28% efficiency 34,720 of them or 10.16 kWh, are actually used for something productive. Yet, we still created 19.4 pounds of CO2 burning that gasoline. A little math, and the emissions result for our car is 1.90 pounds of CO2 per kWh of work produced. Note that at the other end of the scale – at 15% efficiency - those numbers would be 5.4 kWh of useful work, and 3.59 pounds of CO2 per kWh.

Although this article is not about CO2 per mile (remember, simple), for the sake of reference, both the Chevy Volt, and my Chevy Malibu use approximately .240 kWh for each mile at a </i>steady<i> 60 Miles Per Hour. The Tesla Roadster claims to use .217. The Prius is about .220. Your dumptruck, I don't know. However, we do not need to factor in the relative mileage of different vehicles to determine the absolute CO2 emissions potential. Replacing a similar gasoline vehicle, with a similar electric vehicle will still result in similar energy needs.

The answer please!

The results show that burning gasoline in an ICE powered car creates anywhere from 21% to 58% more CO2 than getting the same amount of energy from our electric grid for the same size and shape car. And, that point is important.

The other way many of these studies tend to confuse the issue is trying to relate everything to mileage – like pounds per mile. That is truly putting oranges in an apple barrel. The matter of mileage is irrelevant when comparing similar size and shape vehicles where the only difference is the source of power. They will both use the same amount of energy, which here is expressed in Kilowatt-hours. Regardless of whether they are powered by electricity, gasoline, or soda pop. The difference lies in the source of the power.

Now granted, if you replace your dump truck with a compact car, you will be emitting less CO2, but that is true of either power source, because your small car uses less energy than a dump truck. The reverse would also be true, although good luck finding an electrically powered dump truck.

It should be clear by now that by replacing a gasoline powered ICE vehicle with an electric one, for the same amount of power at the wheels, we would reduce CO2 GHG emissions by anywhere from 21% to 58% - even with our existing electrical supply. And, in fact, those who have bothered to reduce some of the complex, often politically motivated and artistically spun, studies down to their basic conclusions have found exactly the same thing.

March 26, 2010

The internet, as well as the local coffee shop, is full of claims of super secret gadgets that will make your car get 200mpg, or 300. The story is always that the only reason we don’t have these fantastic high mileage cars is that “Big Oil” has somehow managed to suppress every single company who has ever tried to make one, or has “bought out” every device that could magically do this to your car. Well, All of them except for ONE - the person peddling it.

Well, As I have said many times on this blog (and in the coffee shop) for any particular vehicle it takes a fixed, quantifiable amount of energy to push it around. This is based on the most basic laws of physics, the same laws that give us airplanes, microwave ovens, and, yes, the automobile. So, I’m gonna do it again.

Yes, once more, we will look at how much energy it takes to move your vehicle. Our purpose will be to define the maximum MPG.

A vehicle will achieve the best MPG or energy efficiency traveling at a steady speed on level ground. In that condition, the only energy it needs is what is required to overcome aerodynamic drag and rolling resistance (friction).

Our test subject for this week will be My Late Model Chevy Malibu, a mid-sized four passenger sedan. It weighs 3,460 pounds, has a frontal area of 24.1 square feet, and a Coefficient of Drag of .37, Totally Mainstream. To simplify the math, I didn’t do any. I used this calculator: http://ecomodder.com/forum/tool-aero-rolling-resistance.php.

Here is a summary of the results. Remember this is for a steady speed!

Speed

Horsepower

Watts

BTU/min

35 mph

6.15

4586

261

40 mph

7.94

5920

337

50 mph

12.64

9425

536

55 mph

15.66

11677

664

60 mph

19.17

14295

813

65 mph

23.23

17322

985

70 mph

27.87

20782

1182

80 mph

39.12

29171

1660

Before we continue, look at that chart closely. Note how the power required goes up rapidly? Aerodynamic drag is the largest force opposing your movement at any reasonable speed. That drag increases with the square of the speed. Doubling the speed creates four times as much drag. But, interestingly, power requirements increase at the cube of the speed. So that doubled speed will take eight times as much power.

You will also note, I have not mentioned MPG in that chart. It is irrelevant so far. This chart is the amount of power the vehicle needs. It does not matter whether that power comes from a Gasoline or Diesel engine, an electric motor, compressed air, rubber bands or a hamster wheel. The amount of force it needs to keep moving is the same.

Also, before we look at MPG, which implies liquid fuels, lets look at the chart and apply it to an electric car. Now it turns out an electric motor is very efficient, turning about 95% of the electricity fed into it into mechanical power. If you look at the 60 MPH row, you will find maintaining that speed requires 14,295 watts of power. To quantify that as energy consumed, or work, we have to add a time element. So, at 60 MPH over the course of an hour, we will go 60 Miles – duh. In that hour we will consume 14,295 Watt Hours of electricity, or 14.2 Kilowatt-hours (kWh). That works out to .236 kWh per Mile. In an amazing coincidence, this is the same as is claimed for the Chevy Volt in electric mode. The Chevy Volt, is indeed, nearly Identical in size to the Malibu. The Tesla Roadster with a smaller frontal area, and slight better Cd, claims .217 kWh per mile in mixed driving where it’s lower weight is also a factor.

OK, Finally, lets talk about MPG. The calculator I used will give you a MPG figure for each speed based upon the efficiency of the engine and drivetrain. Putting 100% efficiency in it will yield you your 200mpg at 45 mph. (Try it!). At 60 mph, you will get 140 mpg. How did we do that? Well, there are 114,000 BTU’s in a gallon of gasoline, and we are using 813 of them per mile (minute). That is 140mpg with a perfect engine at 60 mph. Right here, we know that a 200 mpg car is impossible at any speed over 45 mph. Even if it were perfect.

The Second law of Thermodynamics puts an upper limit on the efficiency of a heat engine. This is known as the Carnot efficiency. A modern fuel injected, steel, Otto cycle, internal combustion engine – the one in your car, can achieve a range of efficiencies from about 15% at idle, to 35% at it’s torque peak and with a wide open throttle. A diesel will achieve the upper end of that range most of the time. What that means, is in the best case, only 35% of the BTU’s in that gallon of gas are being converted to mechanical energy. The rest is wasted as heat out your radiator and exhaust pipe.

The maximum possible Highway MPG my Chevy Malibu can achieve without violating the laws of thermodynamics is thus about 49 MPG at 60 MPH. The only way to improve this number, is to improve the aerodynamics, reduce the frontal area, reduce the rolling resistance, go much slower, or only drive downhill. The Toyota Prius is currently the highest mileage car in the US with a Highway MPG of 48. It achieves this primarily by having a Coefficient of drag of .25, and an estimated engine efficiency of 30% at 65 mph. Try putting these numbers in the calculator and then changing them. Getting the picture?

But, wait, it’s a hybrid. Isn’t that why?

Well, I’m glad you asked. Remember we are talking steady speed. The Hybrid function recovers energy lost through braking. That energy recovered was only the energy we used to accelerate the vehicle. At a steady speed, the hybrid has no advantage over a non-hybrid. In fact, you will notice the city mileage (stop and go) is actually higher at 51mpg. That is where the Hybrid does it’s work recovering energy.

And, that brings us to our last topic for now – acceleration. My Malibu weighs 3,460 pounds Every time we accelerate that mass to 65 mph it uses approximately 650 BTU’s of energy to do so. Per the laws of physics, that energy is then stored in the mass of the car as kinetic energy until we decelerate (slow down). When we apply the brakes, we convert that kinetic energy to heat which is then lost to the air. This is why stop and go driving normally has much lower MPG numbers.

My Malibu achieves a real world highway mileage of 26 MPG. That means we use a total of 4,384 BTU’s for each mile driven (Remember 75% - 3288 BTUs - are wasted). If we accelerate twice each mile to 65 (using an additional 1300 BTUs), then stop, that consumption will rise to 5,684 BTU’s per mile. Doing the math again puts our resulting mileage at 20 MPG.

I hope I have shed some light on this topic, and on the impossibility of some of the snake oil. While there are various other factors that will change these results in the real world – air temperature, hills, tires, road surface, and specific engine configuration to name just a few, they will not vary by much. And the results for the “perfect” engine will still set a maximum limit on what is achievable.